KR102051198B1 - High-strength, high-toughness steel plate, and method for producing the same - Google Patents

Abstract

Provided is a high strength and high toughness steel sheet having a tensile strength, Charpy impact absorption energy, and a ductile wavefront ratio equal to or more than a specific value. In mass%, C: 0.03-0.08%, Si: 0.01-0.50%, Mn: 1.5-2.5%, P: 0.001-0.010%, S: 0.0030% or less, Al: 0.01-0.08%, Nb: 0.010-0.080 %, Ti: 0.005% to 0.025%, N: 0.001% to 0.006%, Cu: 0.01% to 1.00%, Ni: 0.01% to 1.00%, Cr: 0.01% to 1.00%, Mo: 0.01% to 1.00%, and V: It has 1 or more types chosen from 0.01 to 0.10% and B: 0.0005 to 0.0030%, remainder consists of Fe and an unavoidable impurity, and at the plate thickness 1/2 position, the area ratio of island-like martensite is 3%. The average particle diameter of cementite in bainite is less than or equal to 90% and the area ratio of bainite is less than 0.5 µm.

Description

High strength, high toughness steel plate and manufacturing method thereof {HIGH-STRENGTH, HIGH-TOUGHNESS STEEL PLATE, AND METHOD FOR PRODUCING THE SAME}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high strength, high toughness steel sheet and a method for manufacturing the same, and more particularly, to a high strength, high toughness steel sheet suitable for a line pipe steel material having high strength, high Charpy impact absorption energy, and excellent DWTT performance. will be.

In line pipes used for the transportation of natural gas, crude oil, and the like, the demand for high strength is very high for the improvement of the transport efficiency by high pressure and the improvement of local welding construction efficiency by thinning. In particular, in a line pipe for transporting high pressure gas (hereinafter also referred to as a high pressure gas line pipe), not only the material properties such as strength and toughness required as conventional structural steels, but also materials related to breakdown resistance peculiar to gas line pipes Properties are needed.

Fracture toughness values in conventional structural steels exhibit resistance to brittle fracture and are used as indicators for designing so that brittle fracture does not occur in the use environment. On the other hand, in the high-pressure gas line pipes, suppression of brittle fracture for avoiding large-scale destruction is not sufficient, and suppression of ductile fracture called unstable ductile fracture is also necessary.

This unstable ductile fracture is a phenomenon in which ductile fracture propagates at a velocity of 100 m / s or more in the tube axis direction in a high-pressure gas line pipe, which may cause a large-scale destruction of several kilometers. Therefore, the Charpy impact absorption energy value and the DWTT (Drop Weight Tear Test) test value required for suppressing the unstable ductility fracture obtained from the past actual gas burst test results are specified, and high Charpy impact absorption energy and excellent DWTT characteristics have been required. . In addition, the DWTT test value here means the wavefront transition temperature which will be 85% of a flexible wavefront.

In response to such a demand, Patent Document 1 discloses a bainite main body in which an aggregate structure is developed by setting the cumulative reduction amount at 700 ° C. or lower to 30% or more in a component system that suppresses the formation of ferrite in the air cooling process after the end of rolling. By forming the structure and making the area ratio of the ferrite present in the old austenite grain boundary 5% or less, a post steel sheet for steel pipe material having high Charpy impact absorption energy and excellent DWTT characteristics and a method of manufacturing the same have been proposed. have.

In patent document 2, in the component system which controlled the carbon equivalent (Ceq) to 0.36-0.60, after primary rolling which performs rolling of 40% or more of a cumulative reduction ratio in a non-recrystallization temperature range, after heating to recrystallization temperature or more, Ar ₃ or less transformation point Ar ₃ After cooling to a temperature of 50 ° C. or more, secondary rolling of 15% or more in the cumulative reduction ratio is performed in a two-phase temperature range, and accelerated cooling to 600 ° C. or less from a temperature of Ar ₁ transformation point or more. A method for producing a high strength, high toughness steel pipe material having a high Charpy impact absorption energy and a plate thickness having excellent DWTT characteristics of 20 mm or more is proposed.

In Patent Literature 3, in mass%, C: 0.04 to 0.12%, Mn: 1.80 to 2.50%, Cu: 0.01 to 0.8%, Ni: 0.1 to 1.0%, Cr: 0.01 to 0.8%, and Mo: 0.01 to 0.8% , Hot rolling at a cumulative reduction ratio of 50% or more in an austenitic non-recrystallized steel for steel containing Nb: 0.01 to 0.08%, V: 0.01 to 0.10%, Ti: 0.005 to 0.025%, and B: 0.0005 to 0.0030% and thereafter Ar ₃ transformation point or more temperature range from martensite after cooling generated from the critical cooling rate of more than Ms point or less than 300 ℃ temperature region, online, the heated microstructure of 90% in volume ratio greater than tempering de martensite A method for producing a steel sheet for high tensile line pipes having a high Charpy impact absorption energy and excellent DWTT characteristics, which is a mixed structure of and lower bainite, has been proposed.

In patent document 4, Ar contains the steel containing C: 0.03-0.1%, Mn: 1.0-2.0%, Nb: 0.01-0.1%, P <0.01%, S <0.003%, and O <0.005% by mass%. ₃ +80 subjected to rolling so that they are at the 50% cumulative rolling reduction in the temperature range of ℃ ~ 950 ℃, while such air cooling after, 10 to 30% reduction in the amount of accumulated Ar ~ Ar ₃ temperature range of -30 ℃ ₃ is By manufacturing by rolling, the manufacturing method of the high strength steel plate whose plate | board thickness which has high absorption energy which a separation does not generate | occur | produce using the processed ferrite without a rolling aggregate structure is 15 mm or less is proposed.

In Patent Literature 5, the cumulative reduction ratio in the austenite-free recrystallization temperature range is 50 to 90 for steel having a carbon equivalent of 0.180 to 0.220%, disclosed as Pcm (= C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B). after rolling, such that the%, and 10 to cool to 45 ℃ / sec or more cooling rate of from Ar ₃ -50 ℃ temperature and stops the cooling when the temperature of the steel sheet is a 300 ~ 500 ℃, the surface layer portion and then, the steel sheet by bangraeng The ratio of the island-like martensite in 10% or less, the ratio of the mixed structure of ferrite and bainite in the interior from the surface layer portion is 90% or more, and the ratio of bainite in the mixed structure is 10% or more, bainite Has a thickness of 1 μm or less, a length of the lath of 20 μm or less, and a long diameter of cementite precipitated particles in the bainite lath of 0.5 μm or less. High-strength steel sheet and a manufacturing method having the characteristic masses and weldability has been proposed.

Japanese Unexamined Patent Publication No. 2010-222681 Japanese Unexamined Patent Publication No. 2009-127071 Japanese Laid-Open Patent Publication 2006-265722 Japanese Laid-Open Patent Publication 2003-96517 Japanese Patent Laid-Open No. 2006-257499

By the way, as a steel plate applied to the high pressure gas line pipe etc. of recent years, it is requested | required that it is high strength and high toughness, specifically, tensile strength is 625 Mpa or more, and Charpy impact absorption energy in -40 degreeC is 375 J or more. And the ductile fracture rate obtained by the DWTT test at -40 ° C is required to be 85% or more.

In the patent document 1, since the Charpy impact test in an Example is performed with the test piece extract | collected from the 1/4 position of plate | board thickness, desired structure is not obtained in the plate | board thickness center part with a slow cooling rate after rolling, and deterioration of a characteristic is carried out. In this regard, there is a possibility that the stopping performance against unstable ductile fracture is low as the steel pipe material for the line pipe.

Moreover, in patent document 2, since a reheating process is essential after primary rolling, an on-line heating apparatus is required, and there is a concern about an increase in manufacturing cost and a decrease in rolling efficiency due to an increase in the manufacturing process. In addition, since the Charpy impact test in the Example is performed with the test piece collected from the 1/4 position of the plate | board thickness, in the center of plate | board thickness, there exists a possibility of the characteristic deterioration, and as a steel pipe material for line pipes, There is a possibility that the stopping performance is low.

The technique described in Patent Document 3 is a technique relating to a high strength steel sheet of TS≥900 MPa utilizing tempered martensite, which is very unstable as a steel pipe material for line pipe because the strength is very high but the Charpy impact absorption energy is not necessarily high. There is a possibility that the stopping performance against ductile failure is low. Moreover, since it accelerates cooling to the temperature range below Ms point after rolling, deterioration of a steel plate shape is also concerned. In addition, since an on-line heating device is required, an increase in manufacturing cost and a decrease in rolling efficiency due to an increase in the manufacturing process are also feared.

The technique described in Patent Literature 4 needs to be air cooled until rolling at a temperature range of Ar ₃ to Ar _{3 to} 30 ° C., after pressing down at a cumulative rolling rate of 50% or more in a temperature range of Ar ₃ + 80 ° C. to 950 ° C. or lower. Therefore, rolling time is prolonged for a long time and the fall of rolling efficiency is feared. Moreover, there is no description regarding DWTT test, and it is feared that the propagation stop performance of brittle fracture is inferior.

In the technique described in Patent Document 5, the internal structure from the surface layer portion is substantially a mixed structure of ferrite and bainite in order to obtain high strength and high toughness. However, since the interface between ferrite and bainite is a starting point of ductile cracking or brittle cracking, assuming a more stringent use environment such as -40 ° C, it cannot be said to have sufficient Charpy impact absorption energy. As a steel pipe material, there is a possibility that the stopping performance against unstable ductile fracture is insufficient.

In the technique described in such Patent Documents 1 to 5, the tensile strength is 625 MPa or more, the Charpy impact absorption energy at -40 ° C is 375 J or more, and the ductile wavefront ratio obtained by the DWTT test at -40 ° C is 85%. It was not possible to manufacture stably the steel sheet above.

Therefore, in view of such circumstances, the present invention has a tensile strength of 625 MPa or more, the Charpy impact absorption energy at −40 ° C. is 375 J or more, and the ductile wave ratio obtained in the DWTT test at −40 ° C. is 85% or more. An object of the present invention is to provide a high strength, high toughness steel sheet and a method of manufacturing the same.

MEANS TO SOLVE THE PROBLEM The present inventors earnestly examined about the steel plate for line pipes about the various factors which affect Charpy impact absorption energy and DWTT characteristic. As a result, in the steel plate containing C, Mn, Nb, Ti, etc.,

(1) While controlling the cumulative reduction rate and rolling temperature in the austenite uncrystallized temperature range,

(2) By making the cooling stop temperature just above the Ms point, the structure of bainite can be made as much as possible by reducing the island-like martensite (also referred to as MA).

(3) Furthermore, by maintaining at a temperature of cooling stop temperature ± 50 ° C, the average particle diameter of cementite present in bainite can be suppressed to 0.5 m or less,

It was found that a high strength, high toughness steel sheet having high Charpy impact absorption energy and excellent DWTT characteristics was obtained.

The gist of the present invention is as follows.

[1] In mass%, C: 0.03% or more and 0.08% or less, Si: 0.01% or more and 0.50% or less, Mn: 1.5% or more and 2.5% or less, P: 0.001% or more and 0.010% or less, S: 0.0030% or less, Al : 0.01% or more and 0.08% or less, Nb: 0.010% or more and 0.080% or less, Ti: 0.005% or more and 0.025% or less, N: 0.001% or more and 0.006% or less, and further Cu: 0.01% or more and 1.00% or less : 0.01% or more and 1.00% or less, Cr: 0.01% or more and 1.00% or less, Mo: 0.01% or more and 1.00% or less, V: 0.01% or more and 0.10% or less, B: 0.0005% or more and 0.0030% or less It is a steel plate which has a component composition which consists of remainder Fe and an unavoidable impurity, The area ratio of island-like martensite in 1/2 position of the plate | board thickness direction of this steel plate is less than 3%, and The area ratio of bainite at 1/2 position in the plate thickness direction is 90% or more, and The high strength and toughness steel plate which has a micro structure whose average particle diameter of cementite which exists in bainite in the 1/2 position of the plate | board thickness direction of a base steel plate is 0.5 micrometer or less.

[2] In addition to the above-mentioned ingredient composition, in mass%, Ca: 0.0005% or more and 0.0100% or less, REM: 0.0005% or more and 0.0200% or less, Zr: 0.0005% or more and 0.0300% or less, and Mg: 0.0005% or more and 0.0100% or less The high strength and toughness steel plate as described in said [1] containing 1 or more types chosen.

[3] The method for producing a high strength, high toughness steel sheet according to the above [1] or [2], wherein the steel slab is heated to 1000 ° C or more and 1250 ° C or less, and then rolled in the austenite recrystallization temperature range, followed by austenite microcrystallization. In the temperature range, rolling is carried out at a cumulative reduction ratio of 60% or more, and the rolling is finished at a temperature of (Ar ₃ points + 50 ° C) or more (Ar ₃ points + 150 ° C) or less, and Ar ₃ points or more (Ar ₃ points + 100 ° C). Accelerated cooling from the cooling start temperature of the following to the cooling stop temperature of Ms or more (Ms point + 100 degrees C) or less at a cooling rate of 10 degrees C / s or more and 80 degrees C / s or less, and the temperature range of the cooling stop temperature ± 50 degreeC The method of manufacturing a high strength, high toughness steel sheet which is maintained at 50 s or more and less than 300 s and then air-cooled to a temperature range of 100 ° C. or less.

In addition, in this invention, all the temperature in manufacturing conditions shall be steel plate average temperature, unless there is particular notice. A steel plate average temperature is calculated | required by simulation calculation etc. from plate | board thickness, surface temperature, cooling conditions, etc. For example, the average temperature of a steel plate is calculated | required by calculating the temperature distribution of a plate thickness direction using a difference method.

According to the present invention, by appropriately controlling the rolling conditions and the cooling conditions after the rolling, the microstructure of the steel can be mainly bainite, and the average particle diameter of cementite present in the bainite can be made 0.5 µm or less. As a result, a steel sheet having a tensile strength of 625 MPa or more, a Charpy impact absorption energy at −40 ° C. of 375 J or more, and a ductile fracture ratio (SA value) obtained by a DWTT test at −40 ° C. of 85% or more was obtained. It is very beneficial to the industry.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

The high strength and high toughness steel sheet of this invention is mass%, C: 0.03% or more and 0.08% or less, Si: 0.01% or more and 0.50% or less, Mn: 1.5% or more and 2.5% or less, P: 0.001% or more and 0.010% or less, S: 0.0030% or less, Al: 0.01% or more and 0.08% or less, Nb: 0.010% or more and 0.080% or less, Ti: 0.005% or more and 0.025% or less, N: 0.001% or more and 0.006% or less, and further Cu: 0.01 % Or more 1.00% or less, Ni: 0.01% or more and 1.00% or less, Cr: 0.01% or more and 1.00% or less, Mo: 0.01% or more and 1.00% or less, V: 0.01% or more and 0.10% or less, B: 0.0005% or more and 0.0030% or less It is a steel plate which contains 1 or more types chosen from the remainder, and remainder is a component composition which consists of Fe and an unavoidable impurity, and the area ratio of island-like martensite in the 1/2 position of the plate | board thickness direction of this steel plate is 3%. Less than, the area ratio of bainite is 90% or more, and in the bainite The average grain size of the cementite that exists has a microstructure more than 0.5 ㎛.

First, the reason for limitation of the component composition of this invention is demonstrated. In addition, the "%" display regarding a component shall mean the mass%.

C: 0.03% or more and 0.08% or less

C forms a bainite subject structure after accelerated cooling, and effectively acts on high strength by transformation enhancement. However, if the amount of C is less than 0.03%, ferrite transformation and pearlite transformation are likely to occur during cooling, so that a predetermined amount of bainite cannot be obtained, and desired tensile strength (≥625 MPa) may not be obtained. On the other hand, when the amount of C exceeds 0.08%, hard martensite is easily formed after accelerated cooling, and the Charpy impact absorption energy of the base material is lowered or the DWTT characteristics may be inferior. Therefore, the amount of C is made into 0.03% or more and 0.08% or less, Preferably you may be 0.03% or more and 0.07% or less.

Si: 0.01% or more and 0.50% or less

Si is an element necessary for deoxidation, and has the effect of improving the strength of steel by solid solution strengthening. In order to acquire such an effect, it is necessary to contain Si 0.01% or more, It is preferable to contain 0.05% or more, It is more preferable to contain 0.10% or more. On the other hand, since the weldability and the Charpy impact absorption energy of a base material fall when Si amount exceeds 0.50%, Si amount shall be 0.01% or more and 0.50% or less. Moreover, it is preferable to make Si amount into 0.01% or more and 0.20% or less from a viewpoint of the softening prevention of the weld part of a steel pipe, and the prevention of the toughness deterioration of a weld heat influence part.

Mn: 1.5% or more and 2.5% or less

Mn, like C, forms bainite main structure after accelerated cooling, and effectively acts to increase the strength by transformation. However, when the amount of Mn is less than 1.5%, ferrite transformation and pearlite transformation tends to occur during cooling, so that a predetermined amount of bainite cannot be obtained and desired tensile strength (≥625 MPa) may not be obtained. On the other hand, when Mn is contained in excess of 2.5%, Mn is concentrated in the segregation part which is inevitably formed at the time of casting, which causes the Charpy impact absorption energy to be lowered or DWTT performance is inferior. It is made into 1.5% or more and 2.5% or less. Moreover, it is preferable to make Mn amount into 1.5% or more and 2.0% or less from a viewpoint of toughness improvement.

P: 0.001% or more and 0.010% or less

P is an element effective for increasing the strength of the steel sheet by solid solution strengthening. However, when the amount of P is less than 0.001%, the effect is not exhibited and the amount of P is made 0.001% or more because it may cause an increase in the Tallinn cost in the steelmaking process. On the other hand, when P amount exceeds 0.010%, toughness and weldability are remarkably inferior. Therefore, the amount of P is made into 0.001% or more and 0.010% or less.

S ： 0.0030% or less

In addition to causing hot brittleness, S is a harmful element present in the steel as a sulfide inclusion and inferior in toughness and ductility. Therefore, it is preferable to reduce S as much as possible, and in this invention, the upper limit of the amount of S is made into 0.0030%, Preferably you may be 0.0015% or less. Although there is no minimum in particular, since extremely low S steelmaking costs increase, it is preferable to set it as 0.0001% or more.

Al: 0.01% or more and 0.08% or less

Al is an element to contain as a deoxidation material. In addition, since Al has a solid solution strengthening ability, it effectively acts to increase the strength of the steel sheet. However, when Al amount is less than 0.01%, the said effect is not acquired. On the other hand, when Al amount exceeds 0.08%, while raising raw material cost, it may be inferior to toughness. Therefore, Al amount may be 0.01% or more and 0.08% or less, Preferably you may be 0.01% or more and 0.05% or less.

Nb: 0.010% or more and 0.080% or less

Nb is effective for increasing the strength of the steel sheet due to precipitation strengthening and quenching effect. Further, Nb has an effect of expanding the unrecrystallized temperature range of austenite during hot rolling, and is effective for improving toughness due to the miniaturization effect of unrecrystallized austenite reverse rolling. In order to acquire these effects, it contains 0.010% or more. On the other hand, when the amount of Nb exceeds 0.080%, hard martensite is easily formed after the accelerated cooling, and the Charpy impact absorption energy of the base material may be lowered or the DWTT characteristics may be inferior. In addition, the toughness of the HAZ portion (hereinafter also referred to as a weld heat influence portion) is remarkably inferior. Therefore, Nb amount is made into 0.010% or more and 0.080% or less, Preferably you may be 0.010% or more and 0.040% or less.

Ti: 0.005% or more and 0.025% or less

Ti forms nitride (mainly TiN) in steel, and when it contains 0.005% or more in particular, it has an effect of refining austenite grains by the pinning effect of nitride, and contributes to securing the toughness of the base metal and the toughness of the weld heat affected zone. In addition, Ti is an element effective for increasing the strength of the steel sheet by precipitation strengthening. In order to acquire these effects, Ti is contained 0.005% or more. On the other hand, when Ti is contained in an amount exceeding 0.025%, TiN or the like is coarsened and does not contribute to the miniaturization of austenite grains. Therefore, the toughness improving effect is not obtained, and the coarse TiN is a soft crack or brittle crack. Since it is the starting point of generation, the Charpy impact absorption energy is significantly lowered, and the DWTT characteristic is remarkably inferior. Therefore, Ti amount is made into 0.005% or more and 0.025% or less, Preferably you may be 0.008% or more and 0.018% or less.

N: 0.001% or more and 0.006% or less

N forms nitride with Ti, suppresses coarsening of austenite, and contributes to improvement of toughness. In order to acquire such a pinning effect, N is contained 0.001% or more. On the other hand, when N amount exceeds 0.006%, when TiN decomposes in a weld part, especially the weld heat influence part heated at 1450 degreeC or more in the vicinity of a melting line, the toughness of the weld heat influence part resulting from solid solution N may be inferior. Therefore, when the amount of N is made into 0.001% or more and 0.006% or less, and the required level with respect to the toughness of a weld heat influence part is high, it is preferable to make N amount into 0.001% or more and 0.004% or less.

In the present invention, one or more selected from Cu, Ni, Cr, Mo, V, and B are included as a selection element in addition to the essential elements.

Cu: 0.01% or more and 1.00% or less, Cr: 0.01% or more and 1.00% or less, Mo: 0.01% or more and 1.00% or less

Cu, Cr, and Mo are all the quenching improving elements, and like Mn, low temperature transformation structure is obtained and contributes to the strengthening of the base metal and the weld heat affected zone. In order to acquire this effect, it is necessary to contain 0.01% or more. On the other hand, when the amount of Cu, Cr, and Mo exceeds 1.00%, respectively, the effect of high strength is saturated. Therefore, when it contains Cu, Cr, and Mo, you may be 0.01% or more and 1.00% or less, respectively.

Ni: 0.01% or more and 1.00% or less

Ni is also a etchability improving element, and since it is not inferior in toughness even if it contains, it is a useful element. In order to acquire this effect, it is necessary to contain 0.01% or more. On the other hand, since Ni is very expensive and the effect is saturated when the amount of Ni exceeds 1.00%, the content of Ni is made 0.01% or more and 1.00% or less.

V: 0.01% or more and 0.10% or less

V is an element effective in forming a carbide and increasing the strength of the steel sheet by precipitation strengthening, and in order to obtain this effect, it is required to contain 0.01% or more. On the other hand, when V amount exceeds 0.10%, carbide amount may become excess and inferior toughness. Therefore, when it contains V, you may be 0.01% or more and 0.10% or less.

B: 0.0005% or more and 0.0030% or less

B segregates at the austenite grain boundary and suppresses ferrite transformation, thereby contributing to the prevention of a decrease in strength particularly in the weld heat affected zone. In order to acquire this effect, it is necessary to contain 0.0005% or more. On the other hand, when the amount of B exceeds 0.0030%, since the effect is saturated, when it contains B, it is made into 0.0005% or more and 0.0030% or less.

Remainder other than the said component consists of Fe and an unavoidable impurity, but if necessary Ca: 0.0005% or more and 0.0100% or less, REM: 0.0005% or more and 0.0200% or less, Zr: 0.0005% or more and 0.0300% or less, Mg: 0.0005% It can contain 1 or more types chosen from 0.0100% or more of things.

Ca, REM, Zr, and Mg have a function of fixing S in steel to improve the toughness of the steel sheet, and exhibiting an effect by containing 0.0005% or more. On the other hand, when Ca contains 0.0100%, REM contains 0.0200%, Zr contains 0.0300% and Mg exceeds 0.0100%, inclusions in steel may increase and deteriorate toughness. Therefore, when it contains these elements, Ca: 0.0005% or more and 0.0100% or less, REM: 0.0005% or more and 0.0200% or less, Zr: 0.0005% or more and 0.0300% or less, Mg: 0.0005% or more and 0.0100% or less.

Next, the microstructure will be described.

The microstructure of the high strength, high toughness steel sheet of the present invention has a tensile strength of 625 MPa or more and a Charpy impact absorption energy at -40 ° C of 375 J or more, and a soft wavefront ratio obtained in a DWTT test at -40 ° C. (SA value) In order to acquire the characteristic which is 85% or more stably, the island-like martensite has the structure mainly having the bainite structure whose area ratio is less than 3%, and the average particle diameter of the cementite which exists in bainite is It needs to be 0.5 micrometer or less. Here, the structure mainly composed of bainite means that the area ratio of bainite is substantially composed of bainite structure with 90% or more. As the remainder structure, not only island-like martensite having an area ratio of less than 3% is allowed, but phases other than bainite such as ferrite, pearlite, and martensite may be included, and these remainder structures are total. If it is 10% or less by area ratio, the effect of this invention can be expressed.

Area ratio of island-like martensite in 1/2 position of plate thickness direction: less than 3%

Since island-like martensite has high hardness and becomes a starting point of ductile cracking and brittle cracking, when the area ratio of island-like martensite is 3% or more, the Charpy impact absorption energy and DWTT characteristics are significantly reduced. On the other hand, when the island-like martensite is less than 3% in area ratio, the Charpy impact absorption energy is not lowered or the DWTT characteristic is not inferior. Therefore, in the present invention, the island-like martensitic in the 1/2 position of the plate thickness direction The area ratio of the trap is limited to less than 3%. It is preferable that the area ratio of the said island-like martensite is 2% or less.

Area ratio of bainite at 1/2 position in plate thickness direction: 90% or more

The bainite phase is a hard phase, and is effective for increasing the strength of the steel sheet by transformation of the transformation structure, and by using the bainite main structure, the high strength can be achieved while stabilizing the Charpy impact absorbing energy and DWTT characteristics. On the other hand, when the area ratio of bainite is less than 90%, the total area ratio of the remaining structures such as ferrite, pearlite, martensite and island-like martensite becomes 10% or more, and in such a composite structure, abnormalities Since the interface is a starting point of the occurrence of soft cracks and brittle cracks, the target Charpy impact absorption energy and DWTT characteristics may not be obtained. Therefore, the area ratio of bainite at the 1/2 position of the plate thickness direction is 90% or more, preferably 95% or more. Here, bainite is lath-shaped bainitic ferrite, and refers to a structure in which cementite particles are precipitated therein.

Average particle diameter of cementite present in bainite at 1/2 position in the plate thickness direction: 0.5 µm or less

Cementite in bainite may be a starting point of ductile cracking or brittle cracking. When the average particle diameter of cementite exceeds 0.5 µm, the Charpy impact absorption energy is significantly lowered, and DWTT characteristics are significantly inferior. However, when the average particle diameter of cementite in bainite is 0.5 µm or less, since these decreases are small and the target characteristic is obtained, the average particle diameter of cementite is 0.5 µm or less, preferably 0.2 µm or less.

Herein, the area ratio of the bainite is mirror-polished to the L cross section (vertical cross section parallel to the rolling direction) from 1/2 position of the plate thickness direction, and then corroded with nital, using a scanning electron microscope (SEM). It can be obtained by randomly observing five fields of vision at a magnification of 2000 times, identifying the tissue by the photographed tissue photograph, and determining the area ratio of each phase such as bainite, martensite, ferrite, and pearlite by image analysis. In addition, the same sample was electrolyzed (electrolyte solution: 100 ml distilled water + 25 g sodium hydroxide + 5 g picric acid) to form island martensite, and then randomized at 2000 times magnification by scanning electron microscope (SEM). 5 visual field observation and the area ratio of island martensite can be calculated | required by image analysis from the image | photographed tissue photograph. In addition, after mirror polishing, cementite was extracted using a selective low-potential electrolytic etching method (electrolyte: 10 vol% acetylacetone + 1 vol% tetramethylammonium chloride methyl alcohol), and then randomly magnified at 2000 times magnification by SEM. The visual observation is carried out, the tissue photograph taken is image-analyzed, and the equivalent circular diameter of cementite particle can be averaged and computed.

In general, since the metal structure of the steel sheet produced by applying accelerated cooling is different in the sheet thickness direction of the steel sheet, the cooling rate is slow from the viewpoint of stably satisfying the target strength and the Charpy impact absorption energy. The structure of 1/2 position (1 / 2t position of plate | board thickness t) of plate | board thickness direction which is difficult to achieve is prescribed | regulated. In other words, if a structure satisfying the above requirements is obtained from the 1/2 position in the plate thickness direction, the 1/4 position in the plate thickness direction can be expected to satisfy the above requirements in the same manner. Even if a structure satisfying the above requirement can be obtained, it cannot be expected that the above requirement is necessarily satisfied at the 1/2 position in the plate thickness direction.

The high strength and high toughness steel plate which has the high absorption energy of this invention which consists of the above has the following characteristics.

(1) The tensile strength of the base metal is 625 MPa or more: In the line pipe used for the transportation of natural gas or crude oil, the strength of the high strength is increased for the improvement of the transport efficiency by high pressure and the improvement of local welding construction by thinning. The demand is very high. In order to meet these demands, in this invention, the tensile strength of a base material is 625 Mpa. Here, tensile strength can be measured by extracting the full thickness tensile test piece whose tensile direction based on API-5L becomes a C direction, and performing a tensile test. Moreover, in the composition and structure of this invention, the tensile strength of a base material can be manufactured without a problem up to about 850 Mpa.

(2) Charpy shock absorption energy at -40 ° C is 375 J or more: In a high-pressure gas line pipe, a high-speed ductile fracture in which a ductile crack caused by an exogenous accident propagates at a velocity of 100 m / s or more in the direction of the tube axis (unstable) Ductile failure) is known to occur, which may lead to large-scale destruction up to several kilometers. In order to prevent such high-speed ductile fracture, high absorption energy is effective. In the present invention, the Charpy impact absorption energy at −40 ° C. is 375 J or more, and preferably 400 J or more. Here, the Charpy impact absorption energy at -40 degreeC can be measured by performing the Charpy impact test based on ASTMA370 at -40 degreeC.

(3) More than 85% of the ductile fracture rate (SA value) obtained by the DWTT test at -40 ° C: in the line pipe used for transporting natural gas, etc., in the DWTT test from the viewpoint of preventing brittle crack propagation. It is desired that the value of the soft fracture rate is high, and in the scope of the present invention, the soft wave rate (SA value) obtained by the DWTT test at −40 ° C. is 85% or more. Here, the soft fracture rate (SA value) by DWTT test at -40 degreeC collects the press-notch type full-thickness DWTT test piece whose longitudinal direction based on API-5L becomes a C direction, and falls at -40 degreeC ( It can obtain | require from the fractured surface by applying the impact bending load by weight).

Next, the manufacturing method of the high strength high toughness steel plate of this invention is demonstrated.

The manufacturing method of the high strength high toughness steel plate of this invention heats the steel slab which consists of above-mentioned component composition to 1000 degreeC or more and 1250 degrees C or less, and after rolling in an austenite recrystallization temperature range, Rolling was carried out at a cumulative reduction rate of 60% or more, and the rolling was finished at a temperature of (Ar ₃ points + 50 ° C) or more (Ar ₃ points + 150 ° C) or less, and a temperature of Ar ₃ points or more (Ar ₃ points + 100 ° C) or less. Accelerated cooling to a cooling stop temperature of at least Ms point (Ms point + 100 ° C or less) at a cooling rate of 10 ° C / s or more and 80 ° C / s or less, and at least 50 s in a temperature range of cooling stop temperature ± 50 ° C. It hold | maintains less than s, and is obtained by performing air cooling to the temperature range of 100 degrees C or less after that.

Slab heating temperature: It is less than 1250 degrees Celsius more than 1000 degrees Celsius

It is preferable to manufacture the steel slab of this invention by the continuous casting method, in order to prevent macro segregation of a component, and you may manufacture by the ingot method. In addition,

(1) After manufacturing the steel slab, the conventional method of cooling to room temperature once and then heating again

In addition,

(2) Direct rolling rolling into a heating furnace in a hot state without cooling and hot rolling; or

(3) Direct rolling, direct rolling, which is hot rolled immediately after performing some heat retention,

(4) Method of charging part of the heating furnace in a high temperature state and omitting part of the reheating (full side charging)

 Energy saving processes such as can be applied without problems.

When heating temperature is less than 1000 degreeC, carbides, such as Nb and V, in a steel slab are not fully dissolved and the strength increase effect by precipitation strengthening may not be acquired. On the other hand, when the heating temperature exceeds 1250 ° C, the initial austenite grains are coarsened, so the Charpy impact absorption energy of the base material may be lowered or the DWTT characteristics may be inferior. Therefore, slab heating temperature shall be 1000 degreeC or more and 1250 degrees C or less, Preferably you may be 1000 degreeC or more and 1150 degrees C or less.

Cumulative reduction in austenite recrystallization temperature range: 50% or more (compatibility range)

After the slab heating is maintained, rolling is carried out in the austenite recrystallization temperature range, whereby austenite is refined by recrystallization, contributing to the improvement of Charpy impact absorption energy and DWTT characteristics of the base material. Although the cumulative reduction rate in a recrystallization temperature range is not specifically prescribed, It is preferable to set it as 50% or more. Moreover, in the component range of the steel of this invention, the minimum temperature of austenite recrystallization is about 950 degreeC.

Cumulative reduction in austenite uncrystallized temperature range: 60% or more

By accumulating 60% or more in the unrecrystallized temperature range of austenite, the austenite grains are stretched, especially in the plate thickness direction, and become fine grains, and thus the Charpy impact absorption energy of the steel obtained by accelerated cooling in this state. DWTT characteristics are good. On the other hand, when the amount of reduction is less than 60%, the fine-graining effect may become inadequate, and the target Charpy impact absorption energy and DWTT characteristic may not be obtained. Therefore, the cumulative reduction ratio in the unrecrystallized temperature range of austenite is 60% or more, and when further toughness improvement is required, it is preferable to be 70% or more.

Rolling end temperature: (Ar ₃ point +50 degreeC) or more (Ar ₃ point +150 degreeC) or less

The large reduction in the high cumulative reduction ratio in the unrecrystallized temperature range of austenite is effective for improving the Charpy impact absorption energy and DWTT characteristics, and the effect is further increased by reducing in the lower temperature range. However, in the low temperature region of less than (Ar ₃ point + 50 ° C), when the aggregate structure develops in the austenite grain, and then accelerated cooling to be the bainite subject structure, the aggregate structure partly inherits to the metamorphic structure. As a result, separation is likely to occur, and the Charpy impact absorption energy is significantly lowered. On the other hand, exceeds (Ar ₃ point +150 ℃), there may be a case where a valid refinement effect in improving the DWTT characteristics that can not be obtained sufficiently. Therefore, the rolling end temperature is less than (Ar ₃ point +50 ℃) than (Ar ₃ point +150 ℃).

Disclosure of the accelerated cooling temperature of cooling: Ar ₃ point or higher (Ar ₃ point +100 ℃) below

If the cooling start temperature of accelerated cooling is less than Ar ₃ point, in the air cooling process after hot rolling to the start of accelerated cooling, a cornerstone ferrite may generate | occur | produce from an austenite grain boundary, and a base material strength may fall. In addition, when the amount of the formation of the cornerstone ferrite increases, the interface between the ferrite and bainite, which is the starting point of the ductile cracking or brittle cracking, increases, resulting in low Charpy impact absorption energy and inferior DWTT characteristics. On the other hand, if it exceeds the cooling start temperature (Ar ₃ point ℃ +100), since the rolling finishing temperature is also high, there may be a case where a valid microstructure refining effect in improving the DWTT characteristics that can not be obtained sufficiently. Further, if it exceeds the cooling start temperature (Ar ₃ point +100 ℃), even if the air-cooling time until after rolling end, initiating the accelerated cooling slightly, there is the case that the recovery or grain growth of austenite progress, the base material toughness is lowered It may become. Therefore, the cooling start temperature of accelerated cooling shall be Ar ₃ points or more (Ar ₃ points + 100 degreeC) or less.

Cooling rate of accelerated cooling: More than 10 degrees Celsius / s, It is less than 80 degrees Celsius / s

When the cooling rate of accelerated cooling is less than 10 degrees C / s, ferrite transformation may generate | occur | produce during cooling, and base material strength may fall. In addition, when the amount of ferrite produced increases, the interface between ferrite and bainite, which is the starting point of ductile cracking or brittle cracking, increases, resulting in lower Charpy impact absorption energy and inferior DWTT characteristics. On the other hand, when it exceeds 80 ° C / s, martensite transformation occurs especially in the vicinity of the steel plate surface layer, and the base metal strength increases, but the Charpy impact absorption energy of the base material is remarkably low, and the DWTT characteristics are remarkably inferior. Therefore, it is preferable to make the cooling rate of accelerated cooling into 10 degrees C / s or more and 80 degrees C / s or less, and to set it as 20 degrees C / s or more and 60 degrees C / s or less. In addition, a cooling rate points out the average cooling rate which divided | divided the difference of cooling start temperature and cooling stop temperature by the time required.

Cooling stop temperature of acceleration cooling: More than Ms point (Ms point +100 degrees Celsius) or less

When the cooling stop temperature of the accelerated cooling is less than the Ms point, martensite transformation occurs and the base material strength increases, but the Charpy impact absorption energy of the base material is remarkably low, and DWTT characteristics are remarkably inferior, in particular, the steel plate surface layer. In the vicinity, the trend becomes significant. On the other hand, when cooling stop temperature exceeds (Ms point +100 degreeC), in the air cooling process after cooling stop, the island-like martensite accompanying coarse cementite and bainite transformation will produce | generate, and Charpy shock absorption energy will become low, and DWTT There are cases where the characteristics are inferior. Therefore, it is preferable to make the cooling stop temperature of accelerated cooling into Ms point or more (Ms point +100 degrees C or less), and to make Ms point or more (Ms point +60 degrees C or less).

Maintenance after accelerated cooling: 50 s or more and less than 300 s in the temperature range of cooling stop temperature ± 50 ° C

Holding conditions after accelerated cooling are appropriately controlled in order to control the average particle diameter of cementite present in bainite and to obtain high Charpy impact absorption energy and excellent DWTT performance. When the holding temperature after the accelerated cooling is lower than the cooling stop temperature of -50 ° C, carbon dissolved in supersaturation in the bainite transformed by cooling cannot be sufficiently precipitated as cementite, and the Charpy impact absorption energy of the base material is low, and DWTT Characteristics are inferior On the other hand, when the holding temperature exceeds the cooling stop temperature + 50 ° C, cementite in bainite aggregates and coarsens, and the Charpy impact absorption energy of the base material is remarkably lowered, and the DWTT characteristic is remarkably inferior. Therefore, the holding temperature after accelerated cooling shall be cooling stop temperature ± 50 ° C.

When the holding time after accelerated cooling is less than 50 s, carbon dissolved in supersaturation in the bainite transformed by cooling cannot be sufficiently precipitated as fine cementite, resulting in low base metal toughness. On the other hand, when the holding time is 300 s or more, the cementite in bainite coagulates and coarsens, the Charpy impact absorption energy of the base material is remarkably low, and the DWTT characteristic is remarkably inferior. Therefore, the holding time after accelerated cooling shall be 50 or more and less than 300 s.

Air cooling up to the temperature range (room temperature) below 100 ℃

After accelerated cooling and holding at least 50 s or less than 300 s in the temperature range of the cooling stop temperature ± 50 ° C, air cooling is carried out to a temperature range (room temperature) of 100 ° C or less.

Moreover, it is preferable not to reheat after the said accelerated cooling. More specifically, it is preferable not to reheat to 350 degreeC or more.

In addition, in this invention, Ar ₃ point and Ms point shall use the value obtained by calculating using the following formula based on content of each element in each steel raw material. The element symbol in each formula represents content (mass%) of each element in steel. It shall be 0 about the element which does not contain.

Ar ₃ (° C) = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo

Ms (° C) = 550-361C-39Mn-35V-20Cr-17Ni-10Cu-5 (Mo + W) + 15Co + 30Al

The steel plate of this invention manufactured by the rolling process mentioned above is used suitably as a material of a high strength line pipe. In order to manufacture a high-strength line pipe using the steel plate of this invention, it shape | molds in substantially cylindrical shape by U press, O press, etc., or by the press-bend method which repeats 3-point bending, welding of submerged arc welding, etc. By carrying out, it becomes a welded steel pipe and expands so that it may become a predetermined shape. The high strength line pipe manufactured in this way may be coated on the surface as needed, and heat processing for the purpose of toughness improvement etc. may be performed.

Example 1

EMBODIMENT OF THE INVENTION Hereinafter, the Example of this invention is described.

After melting the molten steel which consists of the component composition (residual Fe and unavoidable impurity) shown in Table 1 in the converter, and made it into the slab of 220 mm thickness, it hold | maintained after hot rolling, accelerated cooling, and accelerated cooling shown in Table 2, And the steel plate of 25 mm in thickness were manufactured by air cooling to the temperature range (room temperature) of 100 degrees C or less.

From the thick steel plate obtained by the above, the full thickness tensile test piece whose tensile direction based on API-5L becomes a C direction was taken, the tensile test was done, and yield strength (YS) and tensile strength (TS) were calculated | required. Moreover, the Charpy impact test collects the Charpy test piece in which the longitudinal direction which has a V notch of 2 mm from the 1/2 position of a plate thickness direction becomes a C direction, and performs the Charpy impact test based on ASTM A370 at -40 degreeC. The Charpy impact absorption energy (vE- ₄₀ degreeC) was calculated | required. In addition, the press-notched full-thickness DWTT test piece whose longitudinal direction based on API-5L becomes C direction is extract | collected, and the soft fracture rate (SA- ₄₀₎ of the fracture | rupture fractured by applying the impact bending load by dropping at _{-40 degreeC. ℃} ) was obtained. Then, the specimens for tissue observation were taken from the 1/2 position in the plate thickness direction, and the following methods were used to determine the structure, area ratio of bainite, island-like martensite and residual structure, and average particle diameter of cementite.

<Tissue observation>

After taking the test piece for tissue observation from the 1/2 position of the sheet thickness direction of the steel plate, mirror-grinding the L cross section (vertical cross section parallel to the rolling direction), and corroding with nital, the scanning electron microscope (SEM) Five visual fields were randomly observed at a magnification of 2000 times, and the tissues were identified by the photographed tissue photograph, and the area ratio of each phase such as bainite, martensite, ferrite and pearlite was determined by image analysis.

Next, after only the island-like martensite appeared by electrolytic etching method (electrolyte: 100 ml distilled water +25 g sodium hydroxide + 5 g picric acid), 5 visual field observations were randomly observed by 2000-times magnification using SEM, and image | photographing is carried out. The area ratio of island martensite at 1/2 position of the plate | board thickness direction was calculated | required from the structure | tissue photograph by image analysis.

In addition, after mirror polishing, cementite was extracted by selective low-potential electrolytic etching method (electrolyte: 10 vol% acetylacetone + 1 vol% tetramethylammonium chloride methyl alcohol), and then randomly observed 5 fields of vision at 2000 times using SEM. And the average particle diameter (circle equivalent diameter) of the cementite in the 1/2 position of the plate | board thickness direction was calculated | required from the image | photographed tissue photograph by image analysis.

The obtained results are shown in Table 3.

From Table 3, the steel sheets of Nos. 2 to 13 are the invention examples in which the component composition and the production method are suitable for the present invention, and the tensile strength (TS) of the base material is 625 MPa or more and Charpy impact absorption energy (vE) at -40 ° C. _{-40 degreeC} ) is 375 J or more, and the ductile fracture rate (SA- ₄₀ degreeC) obtained by the DWTT test in -40 degreeC is 85% or more, and it is a high strength high toughness steel plate which has a high absorption energy. .

On the other hand, since the amount of C in the comparative example No. 1 and the amount of Mn in the comparative example No. 18 were less than the range of the present invention, respectively, the amount of generation of ferrite and pearlite generated during cooling was high, and a predetermined amount of bainite was obtained. And the desired tensile strength TS is not obtained. Since the amount of Nb in the comparative example No. 14 is the amount of Nb, the amount of C in the comparative example No. 15 is the amount of C and the amount of Mn in the comparative example No. 17 exceeds the scope of the present invention, the amount of hard martensite produced after accelerated cooling increases. The desired Charpy impact absorption energy (vE-40 ° C) and the DWTT characteristic (SA-40 ° C) cannot be obtained. In No. 16 of the comparative example, since the amount of Si exceeds the scope of the present invention, a large area ratio of island-like martensite, which is a starting point of ductile cracking and brittle cracking, is generated, and the desired Charpy impact absorption energy (vE- _{40 ° C} ) is achieved. ) And DWTT characteristics (SA- _{40 ° C} ) are not obtained. In No. 19 of the comparative example, since Ti amount exceeded the scope of the present invention, TiN coarsened and became a starting point of ductile cracking and brittle cracking, and desired Charpy impact absorption energy (vE- _{40 ° C} ) and DWTT characteristics (SA _{-40 ° C} ) is not obtained. In No. 20 of the comparative example, since the amount of Ti was less than the scope of the present invention, the micronized effect of austenite grains due to the pinning effect of nitride (TiN) was not obtained, and the desired DWTT characteristic (SA- _{40 ° C} ) was not obtained. . In No. 21 of the comparative example, since the amount of Nb was less than the scope of the present invention, the effect of miniaturization of unrecrystallized rolling was not obtained, and the desired DWTT characteristic (SA- _{40 ° C} ) was not obtained. In addition, since the amount of generation of ferrite and pearlite generated during cooling is large, a predetermined amount of bainite is not obtained, and the desired tensile strength TS is not obtained.

Example 2

After the molten steel which consists of the composition of steel B, F, and K shown in Table 1 (residual remainder Fe and an unavoidable impurity) is melted in a converter, it is made into the slab of 220 mm thickness, and hot rolling, accelerated cooling shown in Table 4, After the accelerated cooling was carried out, a thick steel sheet having a thickness of 25 mm was produced by air cooling to a temperature range (room temperature) of 100 ° C or lower.

The thick steel sheet obtained by the above was subjected to the full thickness tensile test, the Charpy impact test, the press-notched full thickness DWTT test in the same manner as in Example 1, and to yield strength (YS), tensile strength (TS), and Charpy impact. Absorption energy (vE- ₄₀ degreeC) and ductile wavefront rate (SA- _{40 degreeC} ) were measured.

The obtained results are shown in Table 5.

From Table 5, the steel sheets of Nos. 22 to 24, 34 to 36, 39, and 40, which satisfy the manufacturing conditions of the present invention, the component composition and the production method are examples of the invention suitable for the present invention, and the tensile strength (TS) of the base material. Is 625 MPa or more, the Charpy impact absorption energy (vE- _{40 ° C} ) at _{-40 ° C} is 375 J or more, and the ductile wavefront ratio (SA- _{40 ° C} ) obtained in the DWTT test at _{-40 ° C} is 85% or more. It is made of a high strength, high toughness steel sheet having high absorption energy. In addition, since No.23 and No.35 have a cumulative reduction ratio in the unrecrystallized temperature range, the Charpy impact absorption energy (vE- _{40 ° C} ) and DWTT characteristics are caused by the miniaturization of austenite in steel sheets having the same composition. (SA- _{40 ° C} ) is higher.

On the other hand, in No. 28 of the comparative example, since the rolling end temperature and the cooling start temperature were lower than the range of the present invention, the amount of generation of ferrite generated during rolling or cooling was large, so that a predetermined amount of bainite was not obtained, and thus the desired tensile strength. (TS) is not obtained. In addition, the separation due to the influence of the texture development to occur during rolling, the desired Charpy impact absorption energy (vE _{-40 ℃)} it can not be obtained. In No. 29 of the comparative example, since the cooling rate at the time of accelerated cooling is less than the range of the present invention, the amount of generation of ferrite and pearlite generated during cooling is large, and a predetermined amount of bainite is not obtained, so that the desired tensile strength TS is obtained. I do not lose. In Comparative Example 32 and No. 37 of Comparative Example, since the cooling stop temperature exceeds the range of the present invention, the formation of island-like martensite accompanying coarse cementite or upper bainite transformation in the air cooling process after cooling stop becomes remarkable. Desired Charpy impact absorption energy (vE- _{40 ° C.} ) and DWTT characteristics (SA- _{40 ° C.} ) are not obtained. In the case of No. 31 of Comparative Example and No. 38 of Comparative Example, since the holding temperature time after the accelerated cooling stop exceeded the scope of the present invention, cementite in bainite coagulated and coarsened and became a starting point of ductile cracking and brittle cracking. , Desired Charpy impact absorption energy (vE- _{40 ° C} ) and DWTT characteristics (SA- _{40 ° C} ) cannot be obtained. In No. 41 of the comparative example, since the cooling rate at the time of accelerated cooling exceeds the range of the present invention, the amount of hard martensite produced after the accelerated cooling increases, and the desired Charpy impact absorption energy (vE- _{40 ° C} ) or DWTT characteristics ( SA- _{40 ° C.} ) is not obtained. Comparative Examples No.33 and No.42 of the comparative example is lower than the range of the present invention because the cooling stop temperature, by increasing the amount of martensite, the desired Charpy impact absorption energy (vE _{-40 ℃)} and DWTT characteristics (SA _{- 40 ° C.} ) is not obtained. In Comparative Example No. 30 and Comparative Example No. 43, since the holding temperature time after the accelerated cooling stop was lower than the range of the present invention, carbon solid-dissolved in supersaturation in the bainite transformed by cooling was sufficiently precipitated as fine cementite. Since it cannot be obtained, desired Charpy impact absorption energy (vE- ₄₀ degreeC) and DWTT characteristic (SA- ₄₀ degreeC) are not obtained.

Industrial availability

By applying the high strength, high toughness steel sheet having the high absorption energy of the present invention to a line pipe used for transporting natural gas, crude oil, etc., it is possible to improve the transport efficiency by high pressure or improve the local welding construction efficiency by thinning. You can contribute greatly.

Claims (3)

In mass%,
C: 0.03% or more and 0.08% or less,
Si: 0.01% or more and 0.50% or less,
Mn: 1.5% or more and 2.5% or less,
P: 0.001% or more and 0.010% or less,
S ： 0.0030% or less
Al: 0.01% or more and 0.08% or less,
Nb: 0.010% or more and 0.080% or less,
Ti: 0.005% or more and 0.025% or less,
N: 0.001% or more and 0.006% or less
Containing, in addition
Cu: 0.01% or more and 1.00% or less,
Ni: 0.01% or more and 1.00% or less,
Cr: 0.01% or more and 1.00% or less,
Mo: 0.01% or more and 1.00% or less,
V: 0.01% or more and 0.10% or less,
B: 0.0005% or more and 0.0030% or less
Contains one or more selected from
The balance is a steel sheet having a component composition composed of Fe and unavoidable impurities,
The area ratio of the island-like martensite in the 1/2 position in the plate thickness direction of the steel sheet is less than 3%, and the area ratio of the bainite in the 1/2 position in the plate thickness direction of the steel sheet is 90%. It has a microstructure which is% or more, the average particle diameter of the cementite which exists in the bainite in the 1/2 position of the plate thickness direction of the said steel plate is 0.5 micrometer or less, and tensile strength is 625 MPa or more and 788 MPa or less, -40 degreeC The high strength, high toughness steel sheet whose Charpy impact absorption energy in is 375 J or more and 445 J or less, and the ductile fracture rate obtained by the DWTT test at -40 degreeC is 85% or more and 95% or less. The method of claim 1,
In addition to the component composition, in mass%,
Ca: 0.0005% or more and 0.0100% or less,
REM: 0.0005% or more and 0.0200% or less,
Zr: 0.0005% or more and 0.0300% or less,
Mg: 0.0005% or more and 0.0100% or less
High strength, high toughness steel sheet containing at least one member selected from. In the manufacturing method of the high strength high toughness steel plate of Claim 1 or 2,
The steel slab is heated to 1000 ° C or higher and 1250 ° C or lower,
After rolling in the austenite recrystallization temperature range,
In the austenite unrecrystallized temperature range, rolling with a cumulative reduction of 60% or more,
The rolling is finished at a temperature of (Ar ₃ points + 50 ° C.) or more (Ar ₃ points + 150 ° C.) or less,
Accelerated cooling from a cooling start temperature of at least ₃ Ar ( ₃ points of + 100 ° C.) to a cooling stop temperature of at least Ms (Ms + 100 ° C.) at a cooling rate of 10 ° C./s or more and 80 ° C./s or less. ,
Furthermore, the cooling stop temperature is maintained at 50 s or more and less than 300 s in the temperature range of ± 50 ° C
After that, air-cooled to a temperature range of 100 ° C or less
Manufacturing method of high strength, high toughness steel plate.